1. Field of the invention
The present invention relates to an intermittent drive type magnetic recording apparatus and particularly to an intermittent drive type magnetic recording apparatus capable of automatically adjusting the brake torque of a running system of a magnetic recording medium according to load change.
2. Description of the Prior Art
FIG. 1 is a block diagram of an example of an intermittent drive control portion in a conventional intermittent drive type magnetic recording apparatus. A magnetic tape 1 is held between a capstan shaft 2 and a pinch roller 3 so that it is moved at a certain speed by the rotation of the capstan shaft 2. On the other hand, a feed amount setting circuit 4 is adapted to set an intermittent feed amount of the magnetic tape 1, so that a feed amount setting signal is generated therefrom. The feed amount setting signal is provided as a signal including a prescribed number of pulses (10 pulses, for example) representing a power running period and a prescribed number of pulses (6 pulses, for example) representing a braking period. The feed amount setting signal is supplied to an intermittent drive signal generating circuit 5 and to one input of an error detector 6. Based on the feed amount setting signal, the intermittent drive signal generating circuit 5 generates sequentially a run command signal and a brake command signal for intermittently running the magnetic tape 1. The output of the intermittent drive signal generating circuit 5 is supplied to a motor drive circuit 7. The motor circuit 7 comprises an amplifier, etc., and generates sequentially a driving voltage and a braking voltage based on the command signals from the intermittent drive signal generating circuit 5. An output of the motor drive circuit 7 is applied to a capstan motor 8. The capstan motor 8 has a rotating shaft connected to a pulley 10 through a belt 9. The pulley 10 is fixed to the outer circumference of the capstan shaft 2.
In addition, an angle sensor 11 is fixed to the capstan shaft 2. The angle sensor 11 generates an angle signal corresponding to the rotating angle (the rotating distance) of the capstan shaft 2. As the angle sensor 11, the technique described in Japanese Patent Laid-Open Gazette No. 122909/1984, for example, may be adopted. Briefly stated, the angle sensor described in this gazette comprises a fly wheel 111 attached to the capstan shaft 2 as shown in FIG. 2. Along the outer circumference of the fly wheel 111, a plurality of notches 112 each having a fixed width are provided at equal intervals. On the other hand, over the circumferential portion of the fly wheel 111, light emitting elements 113, 114 and 115 are disposed so that they are in a phase relation with the notches 112 in which they deviate from one another by 1/3 of the width of each notch 112. Under the circumferential portion of the fly wheel 111, light receiving elements 116, 117 and 118 are disposed opposite the light emitting elements 113, 114 and 115. When fly wheel 111 rotates, three angle signals (pulse signals) C.sub.1, C.sub.2 and C.sub.3 are obtained from the light receiving elements 116, 117 and 118, the faces thereof being deviated by 1/3 of each face as shown by (a) to (c) in FIG. 3, respectively. The three angle signals C.sub.1, C.sub.2 and C.sub.3 thus obtained undergo logical operation, whereby the angle range corresponding to a cycle T shown in FIG. 3 can be detected by dividing the angle range into six regions I VI. Thus, detection of a rotational face can be detected with high precision.
The three angle signals C.sub.1, C.sub.2 and C.sub.3 provided from the angle sensor 11 are supplied to a position detector 12. The position detector 12 comprises a matrix circuit for applying logical operation to the output of the angle sensor 11. The position detector 12 detects a moving amount of the magnetic tape 1 and provides a position signal. The position signal is provided as a pulse signal having pulses the number of which correspond to the moving amount of the magnetic tape 1. The output of the position detector 12 is supplied to the other input of the error detector 6. The error detector 6 comprises a counter and a comparator, etc., and detects an error as a difference between the feed amount setting signal from the feed amount setting circuit 4 and the position signal from the position detector 12 (which indicates the signal having the larger number of pulses and the difference between the numbers of pulses of the two signals), so that an error signal is generated. The error signal is supplied to the motor drive circuit 7 as a correction signal.
In addition, the capstan shaft 2 comprises a brake 13 for braking capstan shaft 2. Brake 13 may be a mechanical brake with a felt pad attached thereto.
FIG. 4 is a waveform diagram for explaining the operation of the conventional example shown in FIG. 1. FIG. 4(a) represents the output voltage of the motor drive circuit 7, namely, the voltage applied to the capstan motor 8; FIG. 4(b) represents the running speed of the magnetic tape 1; FIG. 4(c) represents the angle signals provided from the angle sensor 11; and FIG. 4(d) represents the position signal obtained from the position detector 11. In addition, FIG. 4(e) represents pulses generated from a rotational drum pulse generator (not shown) (generally called FF pulses, which serve as reference pulses for adjusting the face of the rotational drum); FIG. 4(f) represents a video head writing pulse; and FIG. 4(e) represents an FM video signal written in the magnetic tape 1. In the following, the operation of the above described conventional apparatus will be described, referring to FIG. 4.
First, this apparatus controls intermittent drive operation based on the feed amount setting signal provided from the feed amount setting circuit 4. More specifically, from the feed amount setting circuit 4, 16 pulses for example are generated, the first to the tenth pulses commanding a power running operation and the eleventh to the sixteenth pulses commanding a braking operation. The intermittent drive signal generating circuit 5 makes the motor drive circuit 7 generate a positive drive voltage as shown in FIG. 4(a) in a period corresponding to the first to tenth pulses of the feed amount setting signal so that the positive drive voltage is applied to the capstan motor 8. In consequence, the capstan motor 8 rotates in the forward direction. The rotation of the capstan motor 8 is transmitted to the pulley 10 through the belt 9 to rotate the capstan shaft 2 in the forward direction. As a result, the magnetic tape 1 moves by the pressing operation of the pinch roller 3. The running speed of the magnetic tape 1 increases gradually as shown in FIG. 4(b).
In this case, the write timing t.sub.0 is determined at a position shown by an arrow in FIG. 4(e) based on the FF pulses shown in FIG. 4(e) and the video head writing pulse shown in FIG. 4(f). In one frame period (1/30 sec) after this timing t.sub.0, the FM video signal is really written by the video head.
Subsequently, when the intermittent drive signal generating circuit 5 detects the tenth pulse of the feed amount setting signal, a negative drive voltage is generated from the motor drive circuit 7 and applied to the capstan motor 8. The rotation of the capstan motor 8 is electrically braked by the application of the negative drive voltage. Consequently, the running speed of the magnetic tape 1 gradually decreases as shown in FIG. 4(b). At this time, mechanical braking by the brake 13 is also applied. By this mechanical braking, the capstan shaft 2 can be rapidly braked with reverse rotation after braking prevented.
On the other hand, when the capstan shaft 2 rotates, three angle signals C.sub.1, C.sub.2 and C.sub.3 as shown in FIG. 4(c) are provided from the angle sensor 11. These angle signals are supplied to the position detector 12 so that they are converted to a position signal indicating the position of the magnetic tape 1. More specifically, the position detector 12 provides a pulse signal as shown in FIG. 4(d), the number of pulses of this pulse signal indicating the moving amount of the capstan shaft 2, that is, the position of the magnetic tape 1. The position detector 12 is structured so as to generate pulses equal to the number of pulses of the feed amount setting signal from the feed amount setting circuit 4, that is, 16 pulses, in case where intermittent drive operation is performed in an ideal manner.
Thus, intermittent recording of an FM video signal for one frame is completed. This intermittent recording is repeated at intervals of several seconds for example. At the time of recording, an FM video signal of each frame is recorded at a position adjacent the FM video signal of the immediately preceding frame and in appearance, signals of the respective frames are continuously recorded on the magnetic tape 1 (see FIG. 6). More specifically, after recording one frame, the magnetic tape 1 is stopped for several seconds and then the magnetic tape 1 is made to run again to record the subsequent frame. Thus, intermittent drive is controlled so that a blank or an overlap may not be generated between the adjacent frames. For this purpose, the feed amount setting circuit 4 sets a power running period and a braking period for intermittent drive to specified values.
However, the rotating angle of the capstan shaft 2 is generally not precisely coincident with the set value because of the moment of inertia of the capstan system. For example, if the rotation of the capstan shaft 2 is too slow, the position signal from the position detector 12 unavoidably contains 17 or more pulses. If the subsequent frame is written in this state, a blank is generated between this frame and the preceding frame.
To prevent this phenomenon, the error detector 6 compares the feed amount setting signal from the feed amount setting circuit 4 and the position signal from the position detector 12 and determines whether the position signal contains exactly 16 pulses. If the number of pulses of the position signal is other than 16, the error detector 6 applies a correction voltage as shown by the solid line 20 or the dotted line 21 in FIG. 4(a) to the capstan motor 8 through the motor drive circuit 7. More specifically, if the number of pulses of the position signal is larger than 16, a negative voltage (a reverse voltage) as shown by the solid line 20 is applied to the capstan motor 8 and if the number of pulses is smaller than 16, a positive voltage (a forward voltage) as shown by the dotted line 21 is applied thereto. As a result, the capstan motor 8 is made to rotate in the reverse direction or in the forward direction so that correction is made. (In this case, the moving amount is extremely small.)
In the above described conventional apparatus, intermittent operation can be performed correctly only in the initial state (at the time of delivery of the product from a factory). In other words, such an apparatus is delivered in a state in which the correction voltage has been adjusted so that recording can be made continuously without causing a blank or an overlap between the adjacent frames.
However, in a VTR for intermittent recording or reproducing, a brake 13 is necessarily needed for applying rapid braking to the capstan shaft 2. The brake 13 comprises generally a felt pad pressed against the outer surface of a fly wheel as described above. As a result, the braking force of the brake 13 is changed due to gradual pad abrasion. If the braking force of the brake 13 is changed, the magnetic tape 1 is not stopped at a desired position and a position error increases. According to the increase of the position error, the correction amount of the correction voltage 20 or 21 is increased, causing problems as follows.
(1) A long period of time is required for correction and it happens that correction can not be made within one operation period in intermittent drive. In such a case, an overlap or a blank is generated between the adjacent frames.
(2) The amount of the magnetic tape 1 moved backward or forward by correction causes a slack in the magnetic tape 1 and as a result, operation of the VTR is unstable.
As described above, in the case of a conventional apparatus, satisfactory performance cannot be maintained over a long time although a satisfactory performance can be exhibited at the time of delivery of the apparatus. Particularly in the case of a long-period VTR mainly used for monitoring, the VTR is usually operated only in a recording mode for several months and it will hardly be operated in a reproduction mode unless it is necessary to reproduce the recorded scenes when an accident or the like occurs. Therefore, long-term reliability in performance is critical and it is desired to solve the above described problems.